170 THE ELECTRIC LIGHT. 



Above and below we see the two wires conducting the 

 electric current, and connected re- 

 spectively with the charcoal points 

 within the egg-shaped glass. This 

 glass is closed above around the 

 metallic rod passing down through 

 the cap of it. There is an opening 

 in the upper end of this rod, into 

 which the wire from the battery 

 can be inserted and secured by the 

 thumb-screw, which, however, is so 

 small in the engraving as to be 

 scarcely visible. There is a simi- 

 lar connection below with the oth- 

 er battery wire. 

 There is an opening through the stem and the base of 

 the instrument beloAV, by means of which the air may be 

 exhausted the instrument being placed upon the plate of 

 an air-pump for this purpose arid then the opening can be 

 closed by means of the stop-cock, the thumb-piece of which 

 is seen in its proper place on the left-hand side of the stem. 

 With this instrument it is shown that the vividness of 

 the light is not diminished by the absence of air, and, con- 

 sequently, that the source of the heat, by which the parti- 

 cles of carbon are made incandescent, is not combustion, 

 but some mysterious property of the current of electricity 

 to manifest itself under certain circumstances in that form. 

 All these things about the different modes of illumina- 

 tion, as used in light-houses, and a great deal more about 

 the application and use of them, Lawrence explained to 

 John in the lecture which he gave him in the railway car- 

 riage, on their way to Paris, on the day after they crossed 

 the Channel, as described in the last chapter. The lecture 

 was in two parts, of half an hour each, with an interval of 



